This invention relates to bubble (domain) lattice files and, in particular, to a scheme for making bubble lattice files defect tolerant.
Any number of prior art patents illustrate a lattice memory file utilizing stable two dimensional hexagonal closely packed array of interactive lattice bubbles created in a bubble film and teach that the lattice bubble diameter and lattice spacing are determined by the applied bias field, bubble-bubble interactions and the magnetic constants of the bubble film. See, for example, the U.S. Pat. Nos. to Mehta, et al. 4,034,357, to Lin, et al., 4,052,711, Hu, et al., 3,953,842 and others. The foregoing art also describes coding schemes utilizing different kinds of domain walls to provide digital information, viz, the U.S. Pat. No. to Hsu, 3,996,577 and also disclose means for confining the lattice bubbles and for limiting their travel to specific directions, viz, the U.S. Pat. Nos. to Eggenberger, 4,028,685, (rails), to Voegeli, 4,001,796 (rails and stripe domains) also Voegeli, 2,930,244 (rails and stripe domains) and to Rosier, 4,040,038 (stripe domains). Also, this prior art teaches means for accessing information contained in the lattice file, viz, the U.S. Pat. Nos. to Rosier 4,040,038 supra, also Rosier 3,913,079 and the Voegeli patents, supra. This, conventionally, is accomplished by moving the lattice bubbles transversely of the rows for a reading and writing function. This transverse movement is called column accessing and to do this, lattice bubbles are moved to an area called an access column defined by column oriented conductors, are separated from other lattice bubbles and are then moved serially in the columns to a read station where the information is sensed according to the selected coding scheme. Writing is accomplished in a similar manner, but opposite to the read operation, in that bubbles are generated at a write station and moved serially in the access column to the lattice file. Movement of the lattice bubbles in the access columns is accomplished in two ways. One way is to pulse a series of conductors located transverse of the access column conductors to create a magnetic field which reacts with the lattice bubbles and moves them serially. The second way is to use a so-called bubble pump, Rosier U.S. Pat. No. 4,040,038 supra, by which lattice bubbles are simply forced serially in and out of the access column by the introduction of additional bubbles generated at the write station which interact with the lattice bubbles in the access columns.
In the manufacture of the lattice files memories, a great number of chips are formed on a large wafer, each of which contains, among other things, a garnet bubble supporting film, conductor overlay patterns and configurations to define the means for confining the lattice bubbles in the rows, for providing the conductors for column accessing, and for movement of the bubbles in the rows. When such chips are formed on a wafer, usually by a photolithographic process, for a number of reasons, one or more of the chips may be found to be defective so that the lattice bubbles may not be able to be maintained in the desired state for the selected code scheme. This, of course, would introduce error into the information to be stored in the lattice file memory and would ordinarily be a reason for rejecting the entire chip or perhaps a number of chips on a wafer. The larger the rejection rate, of course, increases the larger the increase in the cost of the manufacture of the chips.
There are, of course, a number of defect tolerant schemes for the magnet bubble devices arranged in the well known storage loop configuration; the U.S. Pat. Nos. to Ohnigian, et. al. 4,073,012, to Bogar, et al. 3,792,450, being typical examples, but these schemes are inapplicable in lattice memories since lattice memories depend upon the interaction of the bubbles themselves to maintain integrity whereas the other devices interact with propagate elements and not with each other.
It is therefore an object of this invention to provide a defect tolerant lattice file memory so that lattice file memory chips which would otherwise be discarded can now be used.